x64: Tidy up some handling of sinkable loads (#5840)

This commit refactors a bit about how sinkable loads are handled in the
x64 backend. The intention is to bring most handling around sinkable
loads up to date with the current state of the backend since things have
changed since these were originally introduced, namely automatic
conversions between types in ISLE. For example the `Value` type can be
automatically converted to `RegMem` to perform load sinking, but some
rules are still explicitly doing matching themselves.

Here I've removed explicit handling of immediates and sinkable loads
when they're the right-hand-side of an operation. These cases are
already handle by the "base case" when converting a `Value` to a
`RegMemImm`. Instead only rules explicitly for left-hand-side immediates
and sinkable loads remain. This helps cut down on the number of explicit
rules needed.

Additionally in the same manner that `Value` can be automatically
converted to `RegMem` I've added automatic conversions from
`SinkableLoad` to `RegMem` and the various other newtypes. This helps
cut down a bit on rule verbosity where `sink_load_*` is largely no
longer necessary.

cranelift/codegen/src/isa/x64/inst.isle

@@ -4301,6 +4301,10 @@
 (convert IntCC CC intcc_to_cc)
 (convert AtomicRmwOp MachAtomicRmwOp atomic_rmw_op_to_mach_atomic_rmw_op)
 
+(convert SinkableLoad RegMem sink_load)
+(convert SinkableLoad GprMemImm sink_load_to_gpr_mem_imm)
+(convert SinkableLoad XmmMem sink_load_to_xmm_mem)
+
 (decl reg_to_xmm_mem (Reg) XmmMem)
 (rule (reg_to_xmm_mem r)
       (xmm_to_xmm_mem (xmm_new r)))